Snowball Earth: The Story of the Great Global Catastrophe That Spawned Life As We Know Itby Gabrielle Walker269pp, Bloomsbury, £16.99

In the Blink of an Eye: The Cause of the Most Dramatic Event in the History of Lifeby Andrew Parker316pp, The Free Press, £18.99

The discovery of deep time, which first gripped the Victorians, still seems a revelation. Forget any mythical past or future. Our own planet has a history stretching back, as we now reckon, 4,500 million years. Trying to grasp this is like wandering outside the limits of a small town wrapped up in its own affairs and discovering the brink of an abyss: frightful but fascinating.

But if a human life seems insignificant in comparison, a human mind can still explore the whole span. We want to turn deep time into history, to know what happened, when, and why. A tall order when you need evidence of how things were hundreds or thousands of millions of years ago, but not impossible.

Both these books home in on events between 500 and 600 million years ago. If fossils can be relied on, two crucial things happened to life on Earth in that time, after a couple of billion years in which the nearest evolution got to making life interesting was coating undersea rocks with slime. The first complex, multi-cellular organisms appeared. Then, a host of new forms evolved, creatures with skeletons, spines, teeth and other solid parts never seen before. This exuberant proliferation of new kinds, first popularised in Stephen Jay Gould's Wonderful Life, is firmly established as life's big bang, the so-called Cambrian explosion. But what set it off?

Gabrielle Walker thinks the first stage was a big freeze. Her book follows the geologists and climate researchers who have assembled evidence that a drastic fall in temperature led to an ice-covered globe - the Snowball Earth of her title. Sunlight bounced back into space off the newly reflective surface and the planet stayed buried in ice 20km thick until volcanoes had vented enough carbon dioxide into the air to warm it up again, 100,000 centuries or so later.

This idea, promoted most forcefully by Paul Hoffman of Harvard University, runs against the grain of thinking in Earth science. Geology blossomed in the 19th century on the working assumption that what happened in the past is pretty much the same as what we can see now. That was why deep time came into the picture, because uplift is slow, erosion gradual. Only astonishing duration could account for the rise and fall of mountains. Darwin's thinking about natural selection then used exactly the same assumptions to account for the origin of new species through gradual accumulation of small changes.

All terribly scientific, and rather dull. The history of the Earth ends up containing no real events, only processes. This picture, known as uniformitarianism, is still challenged from time to time by maverick catastrophists. Sometimes they win the argument. Almost everyone now believes that asteroid impacts have been crucially important, for example, and not just for the demise of the dinosaurs. But these come from far away. Snowball Earth is a climatic crux, a home-grown catastrophe.

Still, there is good geological evidence for this super ice age, or rather a series of them between 750 million and 590 million years ago. Each time, an advance from the poles induced a positive feedback, more ice reflecting more sunlight and cooling things down still more. Each time, later atmospheric changes eventually saw the ice melt. And, from a geologist's point of view, both changes happened with startling speed.

This would have been stressful for life everywhere, and the idea is that somehow this challenge, which almost certainly wiped out many species, helped induce cells to club together and begin to live in the complex communities we now recognise as higher organisms. The details are entertainingly controversial, partly thanks to Hoffman's relentlessly abrasive attitude to his critics. Walker captures this well and also has a fine time following field geologists from one exotic locale to another. Her book is part vivid scientific travelogue as well as an engaging account of a theory moving from speculation to something much more firmly established.

That is also pretty much the impression British researcher Andrew Parker is trying to create about his explanation for the next big evolutionary event, the Cambrian explosion, the focus of In the Blink of an Eye. Supporters of Snowball Earth use many different kinds of evidence, but Parker outdoes them, with a dazzling array of facts from optics, art history, zoology, geology and paleontology. All are lined up behind his hypothesis that what lit the evolutionary fuse in the Cambrian, exactly 543 million years ago, was the appearance of the first working eye.

The eyes have it because, when you move beyond some light-sensitive patch of cells to an optical sensor linked to enough neurons to interpret the scene, the world revealed immediately divides between hunters and hunted. Light is ever present, and the newly visible contest between predators and prey induced an unparallelled burst of evolutionary creativity.

To help persuade us, Parker enlists a large band of creatures, past and present. Where Walker, the journalist, populates her book with scientific personalities at their most combative, Parker, the scientist, is intent on defending his own hypothesis and settles for simply naming co-workers. But he makes up for it by introducing the features of many weird and wonderful organisms, and their myriad adaptations to light.

As far as we know, the first eye appeared in a trilobite which made its debut right at the beginning of the Cambrian explosion. Ever since then creatures have been iridescing, luminescing, evolving colours for camouflage and display. And always they have been looking about them at what other creatures reveal or try to conceal.

Humans are visual creatures, so maintaining the importance of eyes seems intuitively appealing. But Parker's remarkable command of different lines of evidence makes a fascinating read, even though he lacks Walker's storytelling skills. At the end, his book converges with Snowball Earth, as he still needs to explain why the first eye evolved just when it did. Some increase in available light in the oceans where all life still dwelt might account for it, and there are ways the end of a big freeze could have caused this.

Some popular science books have long lives, but this pair are more snapshots of the state of play in fields where the argument will move on. It is easier to establish that there was severe global cooling than that there was total ice-cover, for example. For one thing, the continents have shifted since the evidence was laid down. There is still scope for a less dramatic slushball Earth, with little or no ice around the equator. And both authors' ideas about crucial stages in evolution are vulnerable to new fossil discoveries.

If either multicellular forms or creatures with eyes are found which predate the best current traces, then there will be some serious rethinking, and some fierce new arguments.

Meanwhile, in their different styles both these books allow the reader to get right inside contemporary scientific debates, and leave a vivid impression of science as a work in progress. For my money, both advance convincing arguments. But there is no need to take my word for it. Much more fun to read them and decide for yourself.

· Jon Turney teaches science communication at University College London. His latest book is Lovelock and Gaia: Signs of Life (Icon Books).